According to Chuck Senetors Book:

The 230 casting blocks have thick cylinder walls and thicker main webbing, if you can find one, their your best choice. They were cast only in mid to late 71. The 72 - 75 dated blocks typically have thick enough cylinder walls to overbore, and should be the next choice. The 76 -78 dated blocks have questionable overbore thickness. Don't throw it away, but be careful with cylinder thickness.